It is known that silk filament fibers, like synthetic fibers, are not given permanent curl shrinkage only by twisting. There has been a proposal in which silk fibers are treated with hydrophobic resin to give it curl shrinkage whereby it is maintained at twisted condition even though it is subjected to heat (Japanese Patent Application Publication No. 16,853/1962.) However, since the silk fibers are covered with resin, the essential properties of silk fibers cannot be obtained. Also, the facilities are obliged to be large and the processes are disadvantageously complicated.
There is proposed a process for producing a curl shrink silk yarn in which silk fibers are scoured by a scouring agent including crude sodium sulfate (Na.sub.2 SO4 10H.sub.2 O). This has been disclosed in Japanese Patent Application Publication No. 14043/1972.) In that process, the crude sodium sulfate serves to prevent scouring, and therefore, when a soap together with the crude sodium sulfate scours the silk fibers, 10% of sericin remains on the silk fibers without removal. This causes the silk fibers to be given a curl shrinkage.
One of the disadvantages of the prior art is that the crude sodium sulfate included in the scouring agent causes the product to lack a uniformity in scouring. Another disadvantage of the prior art is that a predetermined amount of the remaining sericin cannot be maintained because the solubility of the sericin by alkali(soap or sodas) or the like is based on the season of rearing silkworms (spring, summer, autumn and late autumn) or on the breed of silkworms. Accordingly, a product of uniform curl shrinkage cannot be obtained. Another disadvantage of the prior art is to provide a luster and a texture to the scoured silk fibers due to the remaining sericin of the silk fibers. Further disadvantage of the prior art is in loss of the strength and durability of the silk fibers if they are treated at temperature of 148.degree. to 153.degree. C. and under pressure of 3.9 to 4.5 kg/cm.sup.2 as disclosed in the Japanese Patent Application Publication No. 14043/1972, due to sensitivity to the heat and high pressure of the animal fibers.